The present invention relates to plastics bottles and in particular to bottles for carbonated drinks.
In designing bottles for carbonated drinks care has to be taken to provide a structure capable of withstanding the pressures resulting from several volumes of carbonation. This is made more difficult when the ambient temperature is high; partly as a result of the thermoplastic nature of the plastics material and partly as a result of the solubility of carbon dioxide in the beverage decreasing with increasing temperature. In practice it is found that failure of bottles under pressure tends to occur at the base. Typically the plastics material in the base creeps and so is gradually extended. Accordingly in many widely used designs for plastics bottles the base has a domed, generally hemispherical shape like that of a pressure vessel. Although such a shape is able to withstand high pressures with little creep it is not inherently stable and so the base has to be provided with a flat-bottomed outer base cup so that the bottle can stand upright. The outer base cup also accommodates what creep takes place.
To overcome the disadvantages of such designs requiring the use of a separate outer base cup to provide stability it has been proposed to use bottles with a "Champagne" base or a castellated base including a number of projecting feet. To produce a "Champagne" base the bottle is first blown to have a domed base and then, whilst still hot and mouldable the domed base is pushed upwards into the bottle with a round nosed tool. This form of base is particularly popular with PET bottles of small capacity, for example those having a capacity of half a litre or less, and it is this shape which is most commonly used for such bottles. An example of a PVC bottle which is blown into a mould to form it with a similar base is described in GB-A-1237402. Castellated bases are more usually used on bottles of larger capacity, typically a litre or more. Examples of such bottles are described in GB-A-1360107, U.S. Pat. No. 3,935,955, U.S. Pat. No. 4,318,489, EP-A-0028125, and WO86/054,62. Our earlier European application EP-A-225155 also shows such a bottle having seven feet formed in the base which makes it particularly stable. Although such designs have been found to be generally satisfactory in both withstanding pressure without everting and offering good handling properties, it is difficult to ensure that there is sufficient material in the base of the bottle to form the feet without undue local thinning of the walls and to provide the necessary strength. One way in which this can be achieved is by increasing the wall thickness of the plastics material in the bottle as a whole. However the amount of plastics material used to form the bottle is a major factor in determining the price of the bottle and so it is undesirable to increase the amount of plastics material used.
GB-A-1360107 describes a plastics bottle for carbonated drinks which has a side wall and a base formed with a central area surrounded by circumferentially spaced projecting feet separated by substantially parallel-sided straps lying on a domed surface. The central area of the base includes an annular re-entrant ring having a substantial extent in the axial direction of the bottle to buttress the base of the bottle. This re-entrant ring is described as acting as a structural arch to resist the internal pressure within the bottle and it is typically formed by pushing an annular tool upwards against the base of the bottle during its blowing step in a similar fashion to the formation of the recessed "Champagne" type base. In some examples the central area of the base is recessed into the annular ring so that any creep of the base does not result in the central area moving downwards farther than the plane defined by the feet. A similar arrangement with a recessed base is also shown in U.S. Pat. No. 4,318,489.